Ouch. I'd be surprised if there's much in the way of specific resources that will make sense that young, though maybe someone knows some. Many children (ok, including me) think about suicide that young, but surely doing it is rare. I would talk about it in general terms, talk about how to get help if things feel hopeless, mention that brains can get into a state where things look much more hopeless than they are, depression as chemical imbalance etc. But avoid "while the balance of the mind was disturbed" stuff because that's really scary - makes one wonder if one's own brain could suddenly trick one into suicide. Might say that people do talk about it that way because it can make them feel better about it than if they think the person had a real choice.
Given that there's depression in my family - and, more, because he was important to me - I've made a point of mentioning to DS my friend who killed himself, to make sure the topic is on the table and he knows I wouldn't be shocked at the very idea. This may be c your chance to let them believe they could talk to you if they ever felt that way.
